The goal of this proposal is to elucidate the cellular mechanisms of activity-dependent plasticity through molecular genetic studies of rodent visual system development. We previously cloned a large number of candidate plasticity genes (CPGs). Preliminary screens identified six whose expression is modulated by light-driven neural activity in rat visual cortex. In the previous grant period we addressed the function of two of these CPGs, cpg15 and cpg2, in the context of visual system plasticity. Our studies show that the cpg15 gene product, CPG15, functions as a membrane-bound growth factor that promotes dendritic growth in neighboring neurons. CPG2 is a structural protein that is localized to dendritic spines and interacts with the actin cytoskeleton. The expression patterns and cellular functions of both proteins are consistent with roles in formation and restructuring of synapses. We propose to continue analysis of cpg15 and cpg2 and their role in visual system development, with the aim of revealing previously unknown molecular details of the mechanisms that underlie activity dependent plasticity. Patterns of cpg15 expression in the visual system suggest that it may play a dual role during development. Early, activity-independent CPG15 expression may contribute to target selection and establishing early patterns of connectivity. Later, activity-dependent CPGt5 expression may contribute to the structural remodeling and synaptic maturation associated with developmental as well as adult plasticity. To test these hypotheses we will generate two lines of cpg15 knockout mice that differ in extent and onset of cpg15 deletion. CREB is a transcription factor essential for activity-dependent plasticity. The cpg15 promoter contains two potential CREB binding sites. To test whether CREB plays a role in cpg15's activity-dependent regulation, we will determine if CREB binds and activates upstream elements in the cpg15 promoter, and study cpg15 regulation in the visual system of CREB knockout mice in vivo. To test the hypothesis that CPG2 may be associated with key developmental steps in maturation of visual cortex, the time course for CPG2's expression and localization will be determined in relation to developmental events such as ingrowth of thalamocortical afferents, maturation of synaptic connections, spine formation, and appearance of pre-and post-synaptic components. To test the hypothesis that CPG2 expression, localization or association with synaptic proteins is regulated by activity, and that this regulation is related to developmental plasticity, we will compare CPG2 expression during normal development with expression after visual manipulations. Comparing the effects of monocular action potential blockade with those of monocular deprivation and dark rearing can discriminate between responses that merely reflect levels of activity and those that are associated with plastic changes.